Annular flow meters have been heretofore suggested and/or utilized (see U.S. Pat. Nos. 3,196,680, 4,638,672 and 1,126,275). These flow meters utilize a central disk or cone mounted in a pipe or conduit to divert fluid flow to the outside of the meter to produce a measurable differential pressure. Annular meters have the advantage of providing free drainage for heavy materials at the bottom of the pipe while at the same time, allowing lighter fluids to pass along the top of the pipe.
Use of such devices often requires that the accuracy, specifications, features, and installation requirements be confirmed with the manufacturer, which in many cases lack documentation as to their accuracy and performance characteristics, and requires a flow test calibration to determine an accurate flow coefficient upon set up. Flow test calibration of such heretofore known devices must be performed on the same fluid and at the same flowing conditions (pipe size and the like) to be of any value, and thus recalibration may also be often required in some facilities. Moreover, due to the wide range of user applications and the limited calibration facilities available to many users, the flow coefficient is quite often based only on the manufacturer's estimated value for various applications. As may be appreciated, such heretofore known annular flow meter designs may be unacceptable to many users due to questionable accuracy and/or requirements for calibration and recalibration.
In some such heretofore known annular flow meter designs, structural failure due to flow induced vibration, clogging of the low pressure ports and high signal to noise ratios have been experienced by users due to aerodynamic and fluid dynamic design shortcomings. The effects of boundary layers and attention to ideal flow theory in such meter designs have also often been overlooked.
For example, many of the previous annular flow meter designs have required structural support members located forward of the low pressure sensing ports. Such support members create upstream disturbance that destabilizes the fluid velocity profile and pressure distribution around the meter. Non-aerodynamic shapes utilized in some heretofore known designs produce high-pressure drag, high permanent pressure loss, and an increase in the possibility of destructive resonance and/or significantly greater flow induced vibration created by the alternate shedding of vortices.
Heretofore known annular flow meters have also often located their low pressure ports in the partial vacuum in the rear wake area of the annular flow inducing mechanism, after the fluid separates from the meter body. When thus positioned, ports collect dirt and are thereby subject to clogging, and can produce high signal noise induced by the alternate shedding of the vortices. The failure in most annular flow meter designs to control boundary layers (the layer that exists between the surface of the meter and the free stream velocity of the fluid, flow of which can be laminar or turbulent), permitting transition from laminar to turbulent flow adjacent to the pressure measuring port(s), results in unpredictability of pressure distribution and its corresponding flow coefficient.
Finally, heretofore known and/or utilized annular flow meters (as well as other venturi-type differential flow meters) are often unable to measure low velocity/Reynolds Numbers flow rates. Typically, such meters have an operation range (maximum flow/minimum flow), or turndown, of about 4 to 1. This is because, at low velocities, increase in laminar boundary layer thickness in the throat adjacent to the low pressure port causes the sensed pressure at the low pressure port to deviate from the true static pressure, and causes a corresponding rapid lowering of the flow coefficient. This results in a loss of accuracy when the flow coefficient deviates outside of the required accuracy band, and thus limits operating range.
As may be appreciated, further improvement in the field of annular flow measurement systems and methods could be utilized.